Our objective is to map genes by annealing iodine 125 labeled RNA or DNA to human chromosomes and to detect the molecular hybrid by autoradiography. This method of in situ hybridization has enormous potential, since theoretically any nucleic acid sequence can be detected in a genome. We plan to locate the genes coding for 5S ribosomal RNA, transfer RNA. When the sensitivity of the method has been improved, attempts will be made to hybridize messenger RNA and other unique sequences.